Multichannel pulse communication system employing complex multivibrator modulators



. M. LEVY 2,454,815

MULTICHANNEL PULSE COMMUNICATION SYSTEM EMPLOYING Nov. so, 1948.

COMPLEX MULTIVIBRATOR-MODULATORS Filed NOV. 10, 1945 4 Sheets-Sheet '1 F/GJ.

COM/"L 5X MuL T/V/B/M 70/? GAT/NG VAL/ES m I 4 f m w R m w 3W M 2 m 0 w m M 0 2,454,815 LQYING 4 Sheets-Sheet 2 LEVY Nov. 30, 1948.

MULTICHANNEL PULSE commumcmxou srs'mu EMF courmax MUL'IIVIBRATOR' uonumrons Filed Nov. 10, 1945 Nov. so, 1948. M M, Y 2,454,815

MULTICHANNEL PULSE CCBMMUNICATION SYSTEM EMPLQYING COMPLEX MULTIVIBRATOR MODULATORS Filed NOV. 10, 1945 4 Sheets-Sheet 3 Inventor MW W 4 7 NOV. 30, 1948. LEVY 2,454,815

NULTICHANNEL PULSE COMMUNICATION SYSTEM EMPLOYING v COMPLEX MULTIVIBRA'I'OR MODULATORS Filed NOV. 10, 1945 4 Sheets-Sheet 4 v F/Ci5.

GA T/A G VALVES Inventor "7W a-alt A51? Attorney Patented Nov. 30, 1948 MULTI-CHANNEL PULSE COMMUNIGATION SYSTEM EMPLOYING COMPLEX MULTL- VIBRATOB 'MODULATORS Maurice Moise Levy, London, England, assignor to Standard Telephones and Cables Limited, London, England, a British company Application November 10, 1945, Serial No. 627,947

. InGreat Britain October 3, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires October 3, 1964 17 Claims. 1

The present invention relates to electric pulse generating circuits of themultivib-rator type suitable for use in electrical pulse communication systems employing time modulation of the pulses. Their application is not however restricted to such systems.

In systems of thewtype specified, the channels are discriminated by the occurrence of. the pulses of the channels duringv respective allotted portions of cyclic period and the. intelligence of a channel may "be transmitted as the. time-phase modulation of the pulses of a channel with-in predetermined time intervals, or as a duration modulation of the pulses, in accordance with the instantaneous amplitude of the signal wave of that channel.

Arrangements for successively and cyclically bringing the channelsinto use are known as distri'butors, there being normally a distributor at the transmitting end and a distributor atthe receiving end of the system working in cos-opera.- tion.

At the transmitter and-receiver, the well known multivibrator circuit arrangement is often em ployed for the production of time-phase modulated pulses at the transmitter for transmission over the communication medium, and for the production of time duration modulated pulses at the receiver under the control of the received time phased modulated pulses for the purpose of obtaining the original modulating signal wave. A multivibra-tor circuit arrangement generally includes at least two electron discharge amplifier devices; and in a multi-channel system the number of such devices is consequently large.

The principal object of this invention is toreduce the number of electron discharge amplifier devices or valves required in the multivibrator circuits of a multi-channel pulse communication system, and this object is attained in accordance with the present invention by providing a complex electric multivibrator comprising a group of two or more amplifying valves having corresponding electrodes connected inparallel, means tor rendering all the said 'val-ves non-conducting, and means for conducting a selected one of the said valves to operate in conjunction with a further amplifying valve separate from the others to guarantee an electric/pulse while maintaining all the other valves of the said group in a non conducting condition.

The invention will be described with reference to the accompanying drawings in which;

Figs. '1 and 2 takentogether .give a schematic. circuit diagram of a transmitter for a time-phase modulation pu se communi ati n system em loying compl x multivibrators ac rding to the inention.

Fi s. 3 and 4 showdiagrams used in explainin the. pera io o "Fi s. 1 and 2; and

Fi 5 shows part of a schematic circuit diaram of a receiver adapted to oper te with the t ansmitter of Figs. 1 and 2- Embo iments of complex multivibrators. accordins to the invention are included in the transmitter for a multichannel pulse transmission syst m shown in Figs. 1 and-2..

The complete circuitof thetransmitter dis vided between the two figures which. should be placed so that Fig. 2 is to the leithand side of Fig. 1.

It will be assumed for clearness that the. sysatem is a 20 channel system, and other particular numerical relations will be assumed, but it will be understoodt'hat-the invention is notrrestricted to such particular. relations.

As the channels have similar circuit elements corresponding thereto, the 20 channels will be denoted in order by the letters of the alphabet from A to T, and accordingly similar elements will be given the same designation numbers, and each will be distinguished by the channel letter corresponding to the element.

A master pulse generator I generates pulses of suitable form at a repetitionfrequency of say 10,000 pulses per second, and supplies them to a distributor 2 from which are obtained the pulses for all the channels. The pulse generator I is preferably a multivibrator of known type and is also preferably-of the typedescribed in the specification of British Patent N 0. 587,940 in which one of the cross couplings between the two multivibrator valves includes a cathode follower ampli fying stage. i

The distributor 2 is preferably a delay networkhaving a number of tappings spaced apart by an interval corresponding to the interval desired for separating the pulses of adjacent channels. This interval will be about 5 microseconds for the particular case under-consideration. The delay net- Work may for example, consist of a plurality of sections compris ng se e inductance 'shunt condensers. Distributors of this type are .described in the U. S. application No. 625,566, filed November 3, 1-945, now abandoned, and in. the British Patent No. 587,939.

.P-ulses from the 'outputend of the delay net- Wor are p terably s pplied to stabilize-the mul tivsibrator in known manner over conductor v,3, a

3 phase inverter 4 being included if this should be necessary.

The channel pulses are generated by three complex multivibrators designated B, 6 and I. These complex multivibrators are all similar, and details of No. 5 only are given. It will be understood that it is not essential that there should be three of these complex multivibrators; this is the preferred number, but other numbers may be used In the particular case chosen for illustration, No. 5 provides pulses for every third channel, namely, A, D, G, J, M, P, and S, No. 6 provides those for B, E, H, K, N, Q, and T and No. '7 provides those for C, F, I, L, O, and R. 7

Figs. 1 and 2 give details for the first few channels and the last, the arrangements for the intervening channels being the same. The complex multivibrator 5 comprises an output valve 8 which is common to all the channels served by this multivibrator, and a plurality of similar input valves 9A, 9D,*9G, etc, each of which corresponds to one of these particular channels. The anodes and cathodes of these channel valves are respectively connected in paralleLand as will be explained later; all these lvalves are initially in a non-'conducting'state, but are operated one at a-tim'e by pulses derived from the distributor 2. Since when one of these valves is operated, all the others are non-conducting, they may be disregarded for the time being. Each of the channel valves 9A, 9D,. 9G, etc., forms in turn a single stroke multivibrator-with the output valve 8. It willonly be necessary therefore todescribe in detail thev action of. the two valves 8 and 9A showrrin Fig.11.v The cathodes of these valves are connected together and tothe .earth terminal l through a cathoderesistance H. The anode of valve 8 is connected: to the positive high tension terminal 12 ,through an anode resistance 3. The control grid is biassed positivelyby means of the resistances i4 and connected in series across the high tension supply terminals, to the junction point .oiwhich the cathode is connected. The anode of the valve 9A is connected to the terminal 12 through: the anode resistance l6, and ispcoupled' to the control grid of the valve 8 through the condenser vll. It will be evident that all the, valves 9A, 9D, 9G, etc., share in common the cathode resistance ,H, the anode resistance [6, and the condenser H. The control grid of the valve 9A is connected through a resistance 18A to a-terminal 19A. The source of modulating signals (not shown) for the channel A is intended tobe connected between terminal ISA and an adjacent earthed terminal 20A. Operating pulses for the valve 9A are applied to the control grid through condenser ZIA. Output time-phase modulated channel pulses are obtained from the anode of the valve 8 through the blocking condenser 22.

The operating pulses for the valve 9A are obtained trom a corresponding gating valve 23A arranged as a. cathode follower. The anode of thisvalve is, connected directly to the positive high tension terminal l2, and, the cathode is connected to earth through a network comprising a resistance 24A connected in parallel with a series combination of the condenser 25A and resistance 26A. The control grid is connected through resistancelZlA to the first tapping A on the distributor 2. The cathode of the valve 23A is connected to the-control grid of the valve 9A'through the condenser 2 IA already mentioned. Referring'now to Fig. 3, the form of'a pulse delivered to the distributor 2 by the master generator I is shown at a... This is a rectangular pulse having a duration a little less than the time interval allowed for thetime-phase variation of any channel pulse. In the example chosen, this width will be a little less than 5 microseconds. A pulse similar to a is obtained from the tapping A and is applied to the control grid of the valve 23A. The variation of the cathode voltage of this valve is shown by the full line b Fig. 3. The trapezoidal form of this pulse is due to the condenser 25A which after the initial application of the leading edge of the pulse a charges relatively slowly through the resistance 26A causing a gradual increase in the cathode potential. On the disappearance of the original pulse, the cathode potential does not fall to zero on account of the charge in the condenser which then discharges relatively slowly as indicated. The value of the cathode resistance H and the potential of the control grid of the valve 8 are chosen so that this valve is in the conducting condition with grid current flowing, and the cathode voltage should be high enough to cut off the valve 9A and all the other channel valves in parallel therewith. When the trapezoidal pulse b (Fig. 3) is applied to the control grid of the valve 9A, (assuming that there is no modulating voltage between terminals HA and '20A), the valve is arranged to begin to conduct at a voltage level 0 approximately cutting the centre of the sloping portion of the trapezoid, this being determined by suitably adjusting the amplitude of the trapezoidal pulse. The corresponding incipient fall of anode voltage is communicated through the condenser I! to the control grid of the valve 8 and reduces the cathode current so that the change is augmented by the resulting fall in cathode voltage of the valve 9A which proceeds rapidly to saturation, the valve 8 being temporarily out off. The condenser 11 then discharges so that the potential of the control grid of the valve 8 returns to, its original value and restores the valve 9A to its out off condition. The time constant of the condenser I! and its associated discharging resistances should be such as to produce from the anode of the valve 8 a positive rectangular pulse of about the same duration as the trapezoidal pulse and the leading edge is then difierentiated to produce a very sharp positive pulse d, Fig. 3. 1

When a modulating voltage is applied between the terminals 19A and 20A, the efiect is substantially to raise or lower the trapezoidal pulse. Thus if the pulse is lowered to the position shown by the dotted lines e, the sloping position of the trapezoid cuts the level 0 nearer the trailing edge, and the channel pulse is emitted later at f. Similarly when the trapezoidal pulse is effectively raised by the modulating voltage the channel pulse is emitted earlier. In this way the channel pulse is time-phase modulated by the signal voltage applied to terminals ISA and 23A.

It will be understood that while the valve 9A is operating, all the other valves 9D, 9G, etc., in parallel therewith remain in the cut off condition because of the high positive voltage applied to their cathodes by the current in the resistance l l. Three channel periods later, a pulse similar to a Fig. 3 is applied from the tapping D of the delay network 2 to the gating valve 23D, and this causes the channel valve 9D to act with the valve 8 to generate a pulse for the channel D exactly as has been described for channel A. The valve 9A now remains cut off.

Again three channel periods later, a pulse a from tapping G of the delay network operates the valve 9G through the gating valve 23G and produces a channel pulse for the channel G in exactly the same way. Similarly all the channel valves of the complex multivibrator 5 are operated in turn from corresponding tappings of thedistributor 2.

The negative pulses applied to the control grid of the common valve 8 from each of the channel valves 8A, 9D, 9G, etc, in turn, will each be followed by a positive tail because of the condenser H which couples the valve 8 to the channel valves. The tail of each pulse will tend tointeriere with the next following pulse, resulting in crosstalk. This is eliminated by the grid current which normally flows in the valve 8, and which cuts off these positive tails. The grid current ceases immediately the next pulse arrives. This method of eliminating crosstalk is fully described in the U. S.- co-pending application No. 620,821, filed October 6, 1945.

The complex multivibrators 6 and l are arranged exactly similarly to the multivibrator 5'. Allthe channel valves are operated by corresponding gating valves arranged in the same way as the valve 23A, and shown as blocks 23B, 23C, etc., the last being 23T. Of these 23E, 23E, etc., and every third gating valve to 2ST are connected to the multivibrator 6 and 23C, 23F, etc, and every third valve therefrom are connected to the multivibrator l. Thus as each pulse is delivered from the generator l to the distributor 2 it operates all the gating valves in order from A to T. The time-phase modulated channel pulses are obtained in rotation from the output valves of the three complex multivibrators and are applied to a mixing circuit 28 of any suitable type, together with pulses of the type a (Fig. 3) obtained from the output of the distributor 2 about a channel period earlier than the tapping A, which pulses are employed as synchronising pulses and are applied to the mixing circuit over the conductor 29. The mixed modulated channel pulses obtained from the output of the mixing circuit 28 are applied to the radio frequency or other equipment (not shown) for transmitting the modulated pulses over a communication medium.

A preferred form of the mixing circuit 28 is that described in United States co-pending application No. 518,329, filed September .24, 1945, which has the desirable property of eliminating crosstalk due to interference between successive channel pulses.

Fig. 4 has been provided to make clearer the manner in which the channel pulses are generated in turn by the three complex multivibrators. At 9 are shown one of the rectangular pulses Z supplied to the distributor 2 by the generator I, and also the delayed pulses obtained in turn from the tappings A to G of the delay network 2. At h are shown the trapezoidal pulses obtained from the complex multivibrator 5, at 7' those obtained from multivibrator 6 and at is those obtained frommultivibrator I. At m are shown the corresponding synchronising pulse and the channel pulses which are obtained at the output of the mixing circuit 28. In Fig. 4 the rectangular and trapezoidal pulses have been shown rather narrower than they would normally be in order to make the figure clear. The width of these pulses would u's'ually'be nearly equal, to a channel period (about 5 microseconds in the example chosen) It will be evident that the greatest saving of valves would be obtained by having only: one complex multivibrator which would have one valve similar to the valve 8, and 20 channel valvessirnilar to 9A. The objection to this is that the rectangular pulses generated by the valve 8A- before differentiation to produce the channel pulses may be too close together so that the tail of one pulse may affect the next pulse. Since in the preferred arrangement employing three multivibrators the pulses are three periods apart,

there will be ample time for the disappearance of the tail. However, any suitable number of these complex multivibrators could be used, and the number of channel valves in each should be the nearest whole number to the quotient of the number of channels by the number ofcomplex multivibrators.

It is evident that the amplitude of thesignal wave applied at terminals [9A and 20A must not be sufiicient to operate the valve 9A by itself in the absence of the trapezoidal pulses. This may be ensured by employing trapezoidal pulses of amplitude which will always be greater than any possible signal amplitude. Alternatively, the

ample be superposed on the trailing edges toensure the operation of the valve, in the manner described in the U. S. application No. 682,958, filed July 11, 1946.

An arrangement very similar to that describedwith reference to Figs. 1 to 4 may be usedat the receiver for separting and demodulating the various channel pulse trains. Three or other suitable number of complex multivibrators are employed to derive time-duration modulated pulses from the received time-phase modulated pulses. Fig. 5 shows the details of the arrangement for the first three channels and is generally similar tov Fig. 1, No figure corresponding to Fig. 2 has been provided, since the arrangement is the same except that the circuit details of the gating valves are as shown in Fig. 5. The three complex multivibrators have been designated 5X, 5X and 1X to indicate that they diifer from 5, B and l of Figs. 1 and 2 only in minor details.

The principal difference is that the fundamental multivibrator circuit is intended to generate time-duration modulated pulses whose leading edges are variable in time and coincide with the time-phase modulated channel pulses and whose trailing edges are fixed in time and coincide with the trailing edges of the corresponding channel gating pulses. Another difference is that a cathode follower valve is employed for one of the cross-connections, the arrangement beingthat described in the British Patent No; 587,940.

The elements which correspond to similar elements in Fig. 1 are given the same designation numbers, and will not be described again in detail, except to point out the differences.

The complex multivibrator 5X comprises the valve 8 common to all the channels served by the. multibrator. This valve is now an input valve instead of an output valve. The anode of the valve 8 is coupled to the control grid of the output valve 9A through a condenser 38 and a valve 3| arranged as a cathode follower with its cathode earthed through the resistance 32. The anode of the valve 3| is directly connected to the positive terminal l2 of the high tension supply; The grid resistance for the valve 3! is 33. The channel valve 9A is in this case a pentode of whichthe screen grid is used as the output electrode. The cathode is biassed positively by its connection to the junction point of the resistances 34A and 35A connected across the high tension supply terminals. The control grid is connected to the cathode of the valve 3|. The screen grid is connected to the positive terminal l2 through the resistance 36A and to a demodulating low-pass filter 31A through a condenser 38A. The demodulated signal waves for channel A are obtained at terminals 39A and 40A.

. It will be understood that the valve 3| is common to all the channels corresponding to the multibrator 5, and a number of additional channel valves (not shown) arranged similarly to SA are connected with their control grids and anodes in parallel respectively with the control grid and anode of 8A,Fig. 5. I

e The gating valve 23A (Fig. is in this case not used as a cathode follower, so an anode resistance MA is provided. In this case also, the pulses supplied by the valve 23A are required to be substantially rectangular, and not trapezoidal, so resistance 26A of Fig. l is not required, and the condenser 25A should be of large capacity to serve as a by-passcondenser for the cathode bias resistance 24A. The valve 23A acts as an ordinary amplifier, and the amplified rectangular pulses are applied from the cathode to the cathode of valve 9A through a blocking condenser 42A.

The complex multivibrators GK and 1X will be the same as SK and will be respectively arranged to serve channels B, E, H etc., and channels C, F, I, etc., valves 23B and 230 are arranged in the same way as 23A, but are connected to later tappings B and C, as shown, and are connected respectively to the complex multivibrators BX and IX. The remaining channels are served in a manner similar to that shown in Fig. 2.

The time-phase modulated pulse trains after demodulation from the carrier wave in conventional apparatus not shown, are applied at 43 in parallel to a selector circuit M and to an amplitude limiter 45. The circuit 44 picks out the synchronising pulses Z (Fig. 4) and applies them through the phase inverter 4 to synchronise the pulse generator 4 which is stabilised by pulses obtained from the output end of the distributor 2 over the conductor 3 as explained with reference to Figs. 1 and 2. The pulses supplied from the generator I to the input end of the distributor are obtained at the channel tappings A, B, C, etc., and are supplied in negative sense to the channel valves such as 9A through the corresponding gating valves such as 23A.

The channel pulses after limitation to a constant amplitude in the limiter 45, are applied in parallel to all the complex multivibrators 5X, SK and 1X. In the case of Ex it is seen that they are applied to the control grid of the valve 8. The valve 9A is biassed by means of the resistances 34A and 35A so that it is cut off and cannot respond to pulses applied tothe control grid from the valve 3 I. Valve 8 is normally in the conducting condition so that its anode voltage is relatively low.

The gating pulses are applied in negative sense from the valve 23A to the cathode of the valve 9A so as to counteract the positive bias, rendering the valve sensitive to pulses from the valve 3|. Nothing happens, however, until the corresponding channel pulse is applied negatively to the valve 8. The anode voltage rises and applies a. positive pulse through the cathode follower valve 3| to the control grid of valve 9A, which valve then proceeds to saturation owing to the coupling through the condenser I! to the control grid of the valve 8. The time constant of the condenser I? and the resistance 14 should be high so that the multivibrator remains in this condition until the disappearance of the gating pulse, when it reverts to the original condition. Thus rectangular pulses will be obtained from the screen grid of the valve 9A, the leading edge of which coincides with the corresponding channel pulses, the trailing edges coinciding with the trailing edges of the gating pulses. These rectangular pulses are therefore time-duration modulated, and the modulating signals are recovered in the usual way by means of the low pass filter 31A.

It will be understood that the other channel valves do not effect the operation of the valve 9A since they will all be cut off until the corresponding gating pulses appear at which times the valve 9A will be cut off.

As explained in the British Patent No. 587,940, the cathode follower valve 31 is introduced in order to provide a greater driving power for the output valve 9A. A similar cathode follower coupling valve may also be included, if desired, in the multivibrator at the transmitter (Figs. 1 and 2).

What is claimed is:

1. A complex electric multivibrator comprising a group of two or more amplifying valves having corresponding electrodes connected in parallel, means for rendering all the said valves nonc0nducting, a further amplifying valve, circuit means coupling said valves to said further amplifying valve for multivibrator operation, and means for conditioning a selected one of the said valves to operate in conjunction with said further amplifying valve to generate an electric pulse while maintaining all the other valves of the said group in a non-conducting condition.

2. A complex electric multivibrator comprising a group of two or more similar amplifying valves, the anode of each valve, and also one other electrode, being connected respectively to the anode and to the other similar electrode of the others of said valves, an additional amplifying valve, means for coupling the output circuit of the said group of valves to the input circuit of said additional amplifying valve for multivibrator operation, means for coupling the input circuit of the said group of valves to the output circuit of the additional valve for multivibrator operation, means for biassing all the valves in the said group so that they are in a non-conducting condition, and means for applying signals to condition the valves in the group one at a time for generating an electric pulse in conjunction with the additional valve.

3. A complex electric multivibrator comprising a group of two or more similar amplifying valves having their anodes and cathodes connected respectively in parallel, an additional output amplifying valve separate from the Valves of the group and having its cathode connected to the cathodes of the said valves, a condenser connecting the anodes of the group valves to ,the ;c,ontrol grid of theadditional valve, a cathode resistance common to all the valves means for biassing the additional valve so that it is in a conducting condition, means for applying a sensitising-pulse to each of thesgroup valves in turn for rendering it in a conducting condition ,With out affecting the other group valves, and means for deriving pulses from the output circuitof the said additional valve.

4. A complex electric multivibrator comprising a groupof two or more similar output amplifying valves having their anodes and control grids connected respectively in parallel, an input amplifying valve, means for coupling the anode of the input valve to the control grids of the output valves, a condenser connecting the anodes of the output valves to the control grid of the input valve, means for biassing each of the output valves beyond the cut-off point, means for biassing the input valve so that it is in a conducting condition, means for applying a train of input electric pulses to the input valve, and means for applying sensitising pulses to condition the output valves one at a time to operate in conjunction with the input valve to generate a pulse in response to a particular one of the input pulses, and means for deriving output pulses separately from the several output valves.

5. A complex multivibrator according to claim 2 including a cathode follower amplifying valve stage serving as one of the said means of coupling.

6. A complex multivibrator according to claim 3 in which the means for biassing the additional valve is adapted to produce a grid current in the said valve.

7. A complex multivibrator according to claim 3 comprising means for applying sensitising pulses to the control grids of the said group valves.

8. A complex multivibrator according to claim 4 in which the means for coupling comprising an amplifying valve arranged as a cathode follower.

9. A complex multivibrator according to claim 4 comprising means for applying the input pulses to the control grid of the input valve and means for applying the sensitising pulses to the cathodes of the output valves.

10. A complex multivibrator according to claim 4 in which the output valves have a grid electrode additional to the control grid, and in which means is provided for deriving the output pulses for the additional grid electrode.

11. Terminal equipment for a multichannel electric pulse communication system comprising one or more complex multivibrators each comprising a group of two or more amplifying valves having corresponding electrodes connected in parallel, means for rendering all the said valves non-conducting, a further amplifying valve, circuit means coupling said valves to said further amplifying valve for multivibrator operation, and means for conditioning a selected one of the said valves to operate in conjunction with said further amplifying valve to generate an electric pulse while maintaining all the other valves of the said group in a non-conducting condition; and a pulse generator and distributor for supplying sensitizing pulses in rotation to the group valves of the said complex multivibrator or multivibrators.

12. A transmitter for a multichannel electric pulse communication system comprising one or more complex multivibrators comprising a group of two or moreaniplifying valves havingcorrespending electrodes connected in parallel, means for rendering all'the said valves non-conducting, a further amplifying valve,circuit means coupling said valves to said further ampliiying'valve for multivibrator operation, and means for conditioning a selected 'one' pf the said valves to operate in conjunction with said further amplifying valve :to generate. an electric ,pulse while maintaining all the other valves of the said group in a non-conducting condition; a master pulse generator adapted to supply regularly repeated masterv pulses to a pulse distributor, means for deriving a plurality of separately timed sensitizing pulses from the distributor in response to each master pulse, means for applying the sensitizing pulses in rotation to the group valves of the said complex multivibrators, means for applying respective channel modulating signals to each of the said group valves, means for deriving time-phase modulated channel pulses from the separate amplifying valves of each complex multivibrator, means for mixing the said channel pulses with synchronizing pulses derived from the said distributor, and means for transmitting the mixed pulses over a communication medium.

13. A transmitter according to claim 12 comprising means for deriving trapezoidal sensitising pulses from the distributor, and means for applying the trapezoidal pulses together with the modulating signals in turn to the control grids of the group valves of the multivibrators in such manneras to cause the multivibrators to generate short time-phase modulated channel pulses corresponding respectively to the trapezoidal pulses.

14. A transmitter according to claim 13 in which the deriving means comprises a plurality of amplifying valves each corresponding to one,

of the channels and each arranged as a cathode follower with a condenser and a resistance connected in series with each other and with the cathode.

15. A receiver for a multichannel electric pulse communication system comprising a plurality of complex multivibrators each comprising a group of two or more amplifying Valves having corresponding electrodes connected in parallel, means for rendering all the said valves non-conducting, a further amplifying valve, circuit means coupling said valves to said further amplifying valve for multivibrator operation, and means for conditioning a selected one of the said valves to operate in conjunction with said further amplifying valve to generate an electric pulse while maintaining all the other valves of the said group in a non-conducting condition; a pulse generator adapted to supply regularly repeated input pulses to a pulse distributor, means for deriving a plurality of separately timed sensitizing pulses from the distributor in response to each input pulse, means for applying the sensitizing pulses in rotation to the group valves of the said complex multivibrators, means for applying a plurality of time-phase modulated channel trains of pulses mixed together simultaneously to the input circuits of all the separate amplifying valves of the complex multivibrator or multivibrators, means for separately deriving a corresponding train of time-duration modulated channel pulses from each of the group valves of the complex multivibrators under the control of the corresponding sensitizing pulses, and means for separately demodulatingthe said trains of time modulated channel pulses.

16. A receiver according to claim 15 in which the pulse generator is adapted to be synchronised by synchronising" pulses transmitted with the modulated channel pulses.

17. A receiver according to claim 15 in which the means for applying the sensitising pulses comprises a plurality of amplifying valves each corresponding to one of the channels, the anodes 12 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,155,210 Young Apr. 18, 1939 2,199,634 Koch May 7, 1940 2,405,237 Ruhlig Aug. 6, 1946 OTHER REFERENCES Review of Scientific Instruments, June1943, pages 171-473. 

